Many primates perform a behavior called urine washing (hereby UW) in which an individual urinates into its palms and wipes the liquid on its soles. We tested four hypotheses on the function of UW (sexual signaling, social signaling, thermoregulation and anxiety displacement) by examining social and environmental factors that contribute to its frequency in a wild population of squirrel monkeys (Saimiri collinsi) in Brazil. We collected data over four months (mating seasons of 2017 and 2018) on three social groups (N=185.5 contact hours). Behavioral data were collected using all-occurrence observations. Ambient temperature and humidity were recorded every 30 minutes while in contact with the groups. UW occurred in a social context in only 3.8% of cases (N=310 UW occurrences), providing no support for the sexual and social signaling hypotheses. Furthermore, the mean juvenile UW rate (1.16 events/hr) was higher than that of adults (0.63 events/hr; t77=-3.31, p=0.001). Neither age nor sex affected the occurrence of juvenile UW (X2 (2, N=106)=0.33, p=0.56). No relationship was found between temperature or humidity and UW rates (p>0.05). Both juveniles and adults urine washed at a higher rate when in stressful situations (t46=6.89; p =0.000001), lending support to the anxiety displacement.